The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for using MRI to measure induced radio frequency (“RF”) currents.
In recent years, development of new MRI techniques for diagnosis and treatment monitoring and guidance, has motivated research into the safety of various electrically conductive devices undergoing MRI. Many implanted medical devices contain electrical conductors that currently contraindicate MRI exams. Furthermore, several interventions could benefit from catheter-based devices with electrical conductors running along their length. Unfortunately such devices, especially those with long linear conductors, have the potential to generate significant and unwanted temperature rises in surrounding tissue during MRI. Temperature increases near the catheter are caused indirectly by RF electrical current induced on the catheter wires by the transmit magnetic field, B1. The heating characteristics of long conductive structures during MRI scanning have been extensively investigated; however, due to the complicated nature of RF heating there lacks a consensus on a generally applicable safety strategy.
Several groups are currently investigating various methods of improving the safety of catheter-based devices, many of which have been shown to effectively mitigate heating under specific circumstances. Generally, before a procedure the safety of a certain device and/or configuration is investigated in vitro using a fiber optic temperature probe. This test method, while accurate, has serious disadvantages. Firstly, testing different configurations requires lengthy repositioning and scanning. Secondly, the probe only measures temperature at one or a few isolated points, meaning that unexpected, dangerous hot spots may not be identified. Finally, this method is not applicable for in vivo applications because the temperature rise that must be induced would result in an unsafe condition for the subject.
To address the measurement duration and inapplicability in vivo, induced current can be measured, enabling a prediction of the specific absorption rate (“SAR”) distribution near the conductor and subsequently the local heating behavior. The two main approaches towards induced RF current measurement are: direct sensing which involves a current transducer on or near the conducting wires; and remote sensing which uses image analysis to determine the current that was present during imaging. Most direct sensing devices suffer from the same limitation as the fiber-optic probe in that they can only measure current at one location. One toroidal current sensor has been developed which can easily be relocated; however, it is too big for in vivo applications, as is the case with all other direct measurement strategies.
The induced RF current flowing on the wire during MRI creates a magnetic field in the vicinity of the wire, at the Larmor frequency (excitation frequency of the magnetic field). This magnetic field couples to the transmit magnetic field, causing a noticeable artifact in both magnitude and phase MR images containing the wire.
One method of detecting induced RF currents on wires during MRI involves the use of reverse polarized transmission and/or reception of the MR signal. By transmitting and/or receiving with reverse polarization, the signal from the wire can be isolated from the forward polarized signal generated by surrounding anatomy. As a result, an image acquired in this manner would be black unless any current were flowing on the wire. This technique provides reliable detection of induced RF currents but has not been used for quantitative measurements. This qualitative technique would contradict some useful and safe exams because there exist situations in which currents could be qualitatively detected yet no risk of significant RF heating exists.
Another strategy of measuring induced RF currents is to analyze the artifact induced in the magnitude MR image. The spatial extent of the artifact is determined directly by the magnitude of induced RF current and thus it can be analyzed to measure current. However the magnitude method of remotely measuring induced current suffers from some limitations. Analyzing the magnitude artifact requires accurate B1 maps acquired with several lengthy scans. Some of these scans require a large flip angle or long pulses and thus high RF power and a greater risk of inducing heating during measurement. Also, variations in signal magnitude due to sources other than the wire can be difficult to remove thereby resulting in inaccurate estimates of the induced current.
An image-based current measurement technique that uses a reverse polarized magnetic field to detect signal only from a wire, and then assigns a safety value to the configuration is disclosed in U.S. Patent Appln. No. 2010/0179763. This technique relies on analyzing artifacts in the magnitude of the image and is incapable of directly quantifying induced current, rather it detects any coupling, safe or unsafe, directly from MR signal intensity.
In another image-based technique described by Venook et al. and van den Bosch et al., artifacts in the magnitude of images are analyzed to quantify induced RF current. This method, however, requires several scans to accurately map the magnitude of the magnetic field. Furthermore, the analysis requires the manual interaction of a user to determine current from the magnitude artifact. This magnitude method of remotely measuring currents cannot be used to perform a rapid, automatic measurement using a single image.
It would therefore be desirable to provide a system and method for measuring the current induced in a conductive structure positioned in the bore of an MRI scanner using an automatic and time efficient technique. With a safe, fast and remote current measurement technique, testing experiments can be performed in a fraction of the time and several diagnostic and procedural MRI scans that are currently avoided could be carried out.